President Trump’s national security adviser, Michael Flynn, has resigned following revelations he spoke with the Russian ambassador about U.S. sanctions during a late December phone call. Flynn now says he misinformed Vice President Mike Pence and top White House officials about contents of the call. The phone call was on December 29—the same day President Obama imposed sanctions against Russia and expelled diplomats in retaliation for the alleged Russian hacking of the U.S. election.

Flynn has claimed the phone call involved only small talk. But U.S. officials who reviewed a transcript of a wiretap of the conversation say Flynn discussed the sanctions—a move that violates a 1799 U.S. law, the Logan Act, which prohibits unauthorized people from negotiating with a foreign government.

The Washington Post reports former Acting Attorney General Sally Yates had warned Trump’s administration weeks ago that Flynn might be lying about the call. Last month, Trump ousted Yates after she refused to defend his Muslim travel ban. On Monday, The Washington Post reported the Justice Department had warned the White House that Flynn’s call and his lies made him a blackmail risk. His resignation Monday came after a tumultuous few days in which Flynn increasingly lost support from Trump and key White House officials, even as members of Trump’s inner circle publicly contradicted each other about Flynn’s fate.

On Monday, counselor to the president Kellyanne Conway went on MSNBC to say Flynn had the “full confidence” of the president. One hour later, White House Press Secretary Sean Spicer told reporters Trump was “evaluating the situation.” On Monday night, Trump accepted Flynn’s resignation and appointed retired Army Lieutenant General Keith Kellogg to be the acting national security adviser.

Many are now asking what, if anything, Trump himself knew about Flynn’s conversation. Longtime New York Times reporter and editor Jim Roberts tweeted Monday, “There is still a HUGE question hanging. Did Trump or Bannon know that Flynn was discussing sanctions with the Russian AMB?” Some are even calling for impeachment hearings.

Flynn has been one of Trump’s most criticized and controversial picks for top positions. Flynn is well known for his anti-Muslim worldview, having called Islam a “cancer” and saying “fear of Muslims is rational.” His son, also named Michael Flynn, was fired as a member of Trump’s transition team after he shared fake news pushed by right-wing websites. Among the conspiracy theories Flynn pushed is one claiming Hillary Clinton and other prominent Democrats were running a child sex ring from the back rooms of a Washington, D.C., pizza restaurant.